This invention relates to a secondary treatment process for the biological purification of wastewater wherein the wastewater is aerated with an oxygen-containing gas in a completely mixed tank in the presence of carrier particles for aerobic microorganisms and in particular wherein the gas feed produces a circulating flow of the wastewater about a horizontal axis extending from the inlet zone to the outlet zone of the tank.
The completely mixed tank is oblong in shape, whereby there is a preferred ratio length: width greater than 3:1, most preferably greater than 5:1.
As is known, with the use of freely suspended carrier particles as the settling substrate for microorganisms, the biomass concentration can be substantially increased even in activated sludge plants, and in this way rapid degradation of waste materials present in the wastewater can be achieved. In this connection, use has recently been made of open-cell foam particles as the carrier particles wherein the specific gravity, the size, and the macropores of these particles are so selected that for a higher rate of mass transfer, the foam particles in the oxygenation tank can be circulated upwardly and downwardly by virtue of the prevailing current and aeration.
One disadvantage however, in operating an aeration tank with the use of such carrier particles is that, with the flow of the wastewater-activated sludge mixture oriented toward the outlet of the aeration tank, entrainment of the carrier particles toward the outlet zone will gradually occur. Thereby a buildup in the concentration of carrier particles occurs at the outlet, the latter usually being equipped with a screen or perforated plate or the like for retaining the carrier particles. As a consequence, in the course of time, the carrier particles will not only clog the outlet but also will no longer be available in a sufficient quantity in the inlet zone of the aeration tank where a high biomass concentration is desirable.